Iron base sintered alloy and method for manufacturing the same

ABSTRACT

An iron base sintered alloy has a chemical composition including at least Fe, Cu and C, and a sintered structure including a residual austenite and a martensite. The sintered structure includes a precipitated Cu element that was dissolved in the martensite and that has been precipitated from the martensite. A method for manufacturing an iron base sintered alloy includes conducting a Cu precipitation treatment for a sintered alloy that has a chemical composition including at least Fe, Cu and C, and a sintered structure including a residual austenite and a martensite to precipitate Cu element dissolved in the martensite.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2017-39441 filed on Mar. 2, 2017, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an iron base sintered alloy and a method for manufacturing the iron base sintered alloy.

BACKGROUND

Conventionally, for a sintered member that is made of an iron base sintered alloy and requires hardness and toughness, Fe—Ni—Cu—Mo base alloy in which Ni element is added in order to increase residual austenite has been employed. However, costs of the iron base sintered alloy of the Fe—Ni—Cu—Mo base alloy tend to be high because Ni element is expensive. Hence, other alloy including less Ni element such as Fe—Cu—C base alloy has been recently considered.

JP 2009-185328 A discloses an iron base sintered alloy in which a sintered body has a hardened structure and high dimension accuracy is achieved without Ni. Specifically, in JP 2009-185328 A, the iron base sintered alloy has a chemical composition including 2.5-3.5% by mass of Cr, 0.4-0.6% by mass of Mo, 0.5-1.5% by mass of Cu, 0.4-0.6% by mass of C, the balance of Fe, and inevitable impurities. The iron base sintered alloy has a quenched structure, as a sintered metal structure, in which a base excluding gas cavities is formed of martensite phase, or a mixed structure of 2-20% by cross-sectional area ratio of bainite phase and the balance of martensite phase.

SUMMARY

When the above described iron base sintered alloy of the Fe—Cu—C base alloy is applied to a member (e.g., a member for an engine) that requires high dimension accuracy and is to be used for a long period of time under a high temperature environment, there is a possibility that chronological change of a dimension of the member occurs.

It is an object of the present disclosure to provide an iron base sintered alloy and a method for manufacturing the iron base sintered alloy capable of restricting chronological change of a dimension of a member even when the iron base sintered alloy is applied to the member that is to be used for a long period of time under a high temperature environment.

According to a first aspect of the present disclosure, an iron base sintered alloy has a chemical composition including at least Fe, Cu and C, and a sintered structure including a residual austenite and a martensite. The sintered structure includes a precipitated Cu element that was dissolved in the martensite and that has been precipitated from the martensite.

According to a second aspect of the present disclosure, a method for manufacturing an iron base sintered alloy includes conducting a Cu precipitation treatment for a sintered alloy that has a chemical composition including at least Fe, Cu and C, and a sintered structure including a residual austenite and a martensite to precipitate Cu element dissolved in the martensite.

According to the first aspect of the present disclosure, the sintered structure includes the residual austenite, the martensite and the precipitated Cu element that was dissolved in the martensite and that has been precipitate from the martensite. Even when a member made of the iron base sintered alloy according to the first aspect of the present disclosure is used for a long period of time under a high temperature environment, chronological change of a dimension of the member is restricted.

According to the second aspect of the present disclosure, the Cu precipitation treatment is conducted to precipitate the Cu element dissolved in the martensite. According to the second aspect of the present disclosure, the iron base sintered alloy capable of restricting chronological change of a dimension of a member even when the iron base sintered alloy is applied to the member that is to be used for a long period of time under a high temperature environment is obtained.

According to a third aspect of the present disclosure, an iron base sintered alloy has a chemical composition including at least Fe, Cu and C, and a sintered structure including a residual austenite, a martensite and a precipitated Cu element precipitated from the martensite. Even when a member made of the iron base sintered alloy according to the second aspect of the present disclosure is used for a long period of time under a high temperature environment, chronological change of a dimension of the member is restricted.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings, and in which:

FIG. 1A is a diagram showing a metallograph image of a sintered structure of a test sample of an experiment 1;

FIG. 1B is a diagram showing a metallograph image of a sintered structure of a test sample of the experiment 1;

FIG. 2A is a diagram showing a metallograph image of a sintered structure of a test sample of the experiment 1;

FIG. 2B is a diagram showing a metallograph image of a sintered structure of a test sample of the experiment 1;

FIG. 3 is a diagram showing absorption spectra of test samples of the experiment 1 obtained by X-ray absorption fine structure analysis;

FIG. 4 is a diagram showing absorption spectra of test samples of the experiment 1 obtained by X-ray absorption fine structure analysis; and

FIG. 5 is a graph showing a relationship between a temperature of a Cu precipitation treatment and variation rate of a dimension.

DETAILED DESCRIPTION Embodiment 1

An iron base sintered alloy of an embodiment 1 will be described. The iron base sintered alloy of the present embodiment has a chemical composition including at least Fe, Cu and C, and has a sintered structure including residual austenite and martensite. In the sintered structure, Cu element that was dissolved in the martensite is preliminarily precipitated. Details will be described hereinafter.

The iron base sintered alloy has the chemical composition including at least Fe, Cu and C. The chemical composition may include other element such as Mo, Ni and the like. Specifically, the iron base sintered alloy may be provided by an iron base alloy including at least Cu and C, an iron base alloy including Cu, Co and Mo, an iron base alloy including Cu, C, Mo and Ni, and the like. More specifically, the iron base alloy may be exemplified by an iron base alloy having a chemical composition including Cu, C, balance of Fe and inevitable impurities, an iron base alloy having a chemical composition including Cu, C, Mo, balance of Fe and inevitable impurities, or an iron base alloy having a chemical composition including Cu, C, Mo, Ni, balance of Fe and inevitable impurities.

Specifically, in the above chemical composition, a content of Cu is equal to or greater than 1.0% by mass and equal to or less than 3.0% by mass. When the content of Cu is equal to or greater than 1.0% by mass, strength of a member is obtained. When the content of Cu is equal to or less than 3.0% by mass, effect of improving the strength and material cost are balanced. Preferably, the content of Cu is equal to or greater than 1.2% by mass and equal to or less than 2.8% by mass. More preferably, the content of Cu is equal to or greater than 1.5% by mass and equal to or less than 2.5% by mass.

Specifically, a content of C is equal to or greater than 0.4% by mass and equal to or less than 1.1% by mass. When the content of C is equal to or greater than 0.4% by mass, quenching property in sintering after molding is easily obtained, and thus strength and hardness of a member are easily obtained. When the content of C is equal to or less than 1.1% by mass, cementite is less likely to be formed in the sintered structure, and toughness is easily obtained. Preferably, the content of C is equal to or greater than 0.5% by mass and equal to or less than 1.0% by mass. More preferably, the content of C is equal to or greater than 0.6% by mass and equal to or less than 0.9% by mass.

Specifically, a content of Mo is equal to or greater than 0.2% by mass and equal to or less than 0.7% by mass. When the content of Mo is equal to or greater than 0.2% by mass, quenching property in sintering after molding is easily obtained, and thus strength and hardness of a member are easily obtained. When the content of Mo is equal to or less than 0.7% by mass, effect of improving strength and material cost are balanced. Preferably, the content of Mo is equal to or greater than 0.3% by mass and equal to or less than 0.6% by mass. More preferably, the content of Mo is equal to or greater than 0.35% by mass and equal to or less than 0.55% by mass.

Since Ni is expensive element, a content of Ni is preferably decreased. However, Ni contributes to formation of residual austenite. Although it is not necessary to actively add Ni, the chemical composition may include Ni. Preferably, the content of Ni is equal to or less than 0.20% by mass. More preferably, the content of Ni is equal to or less than 0.15% by mass. Further preferably, the content of Ni is equal to or less than 0.10% by mass. From viewpoints of decreasing Ni, Ni is preferably included as inevitable impurities.

The iron base sintered alloy has the sintered structure including the residual austenite and the martensite. The sintered structure is observed by etching a cross-sectional surface of the iron base sintered alloy with nital liquid and then observing the cross-sectional surface with a metallograph.

For example, a content of Cu in the martensite is less than 0.5% by mass. The content of Cu in the martensite is measured by energy dispersive X-ray analysis with a transmission electron microscope (e.g., TEM-EDX analysis) with respect to a sample before being used under a high temperature environment.

When the content of Cu in the martensite is less than 0.5% by mass, precipitation of Cu element from the martensite during the usage under the high temperature environment is certainly restricted. Therefore, according to the present embodiment, the iron base sintered alloy capable of restricting the chronological change of the dimension of the member is obtained.

From viewpoints of securing the above effects, the content of Cu in the martensite is preferably equal to or less than 0.4% by mass. More preferably, the content of Cu in the martensite is equal to or less than 0.3% by mass. Further preferably, the content of Cu in the martensite is equal to or less than 0.15% by mass. Further preferably, the content of Cu in the martensite is equal to or less than 0.1% by mass.

In the iron base sintered alloy, the Cu element that was dissolved in the martensite is preliminarily precipitated in the sintered structure. That is, the Cu element that was once dissolved in the martensite, which is generated in the quenching, is preliminarily precipitated in the sintered structure during the manufacturing of the iron base sintered alloy before the iron base sintered alloy is used under the high temperature environment.

It is not necessary that the entire Cu element, which was dissolved in the martensite, is preliminarily precipitated in the sintered structure. The Cu element may partially remain dissolved in the martensite as far as being capable of restricting the chronological change of the dimension of the member during the usage under the high temperature environment.

For example, the Cu element is preliminarily precipitated in the sintered structure in a state of intermetallic compound of Cu. In this case, the precipitated Cu compound generates strain to the circumference and increases hardness in micro order. For example, the intermetallic compound of Cu includes Fe—Cu intermetallic compound. In this case, the Cu element is not precipitated in a state of non-metal inclusion such as oxide, and mechanical strength of the iron base sintered alloy is less likely to be decreased.

In the iron base sintered alloy, the Fe—Cu intermetallic compound shows a peak having an area intensity equal to or greater than 0.125. The area intensity of the peak of the Fe—Cu intermetallic compound is an area intensity obtained by employing a sample before being used under the high temperature environment and being calculated with respect to a peak of the Fe—Cu intermetallic compound shown at an energy absorption edge of K-cell of Cu (i.e., Cu K-edge) by X-ray absorption fine structure (XAFS) analysis.

When the area intensity of the peak of the Fe—Cu intermetallic compound is equal to or greater than 0.125, the iron base sintered alloy capable of restricting the chronological change of the dimension of the member is surely obtained. From viewpoints of securing the above effects, the area intensity of the peak of the Fe—Cu intermetallic compound is preferably equal to or greater than 0.127. More preferably, the area intensity of the peak of the Fe—Cu intermetallic compound is equal to or greater than 0.130. Further preferably, the area intensity of the peak of the Fe—Cu intermetallic compound is equal to or greater than 0.132.

Generally, a peak of an absorption spectrum of the Fe—Cu intermetallic compound is observed in a range equal to or greater than 2.0 angstrom (Å) and equal to or less than 2.5 angstrom (i.e., a range equal to or greater than 0.2 nanometers and equal to or less than 0.25 nanometers).

In the iron base sintered alloy, an area intensity of a peak of Fe—Cu solid solution is equal to or less than 1.620. The area intensity of the peak of the Fe—Cu solid solution is an area intensity obtained by employing a sample before being used under the high temperature environment and being calculated with respect to a peak of the Fe—Cu solid solution shown in an energy absorption edge of K-cell of Cu by the X-ray absorption fine structure analysis.

When the area intensity of the peak of the Fe—Cu solid solution is equal to or less than 1.620, the iron base sintered alloy capable of restricting the chronological change of the dimension of the member is surely obtained. From viewpoints of securing the above effects, the area intensity of the peak of the Fe—Cu solid solution is preferably equal to or less than 1.615. More preferably, the area intensity of the peak of the Fe—Cu solid solution is equal to or less than 1.610. Further preferably, the area intensity of the peak of the Fe—Cu solid solution is equal to or less than 1.605. Further preferably, the area intensity of the peak of the Fe—Cu solid solution is equal to or less than 1.600.

Generally, a peak of an absorption spectrum of the Fe—Cu solid solution is observed in a range equal to or greater than 2.25 angstrom and equal to or less than 2.75 angstrom (i.e., a range equal to or greater than 0.225 nanometers and equal to or less than 0.275 nanometers).

For example, the iron base sintered alloy is applied to a member that requires high dimension accuracy and is to be used under a high temperature environment. For example, the member is exemplified by a member for an engine of an automobile or a member for an air cooling equipment. The member for the engine includes an engine member included in the engine. For example, the member for the air cooling equipment includes a vane for an air conditioner and a compressor, an axle bearing, and the like. The member for the engine is likely to require high dimension accuracy and is likely to be used under a high temperature environment such as 50 degrees Celsius (° C.) to 180 degrees Celsius. When the iron base sintered alloy of the present embodiment is applied to the member for the engine, reliability of automobile is increased.

The iron base sintered alloy of the present embodiment has the structure described hereinabove. Especially, in the iron base sintered alloy of the present embodiment, the sintered structure includes the residual austenite and the martensite, and the Cu element that was dissolved in the martensite is preliminarily precipitated in the sintered structure.

The Cu element that was dissolved in the martensite and that has been precipitated from the martensite may be referred to as a precipitated Cu element. That is, the sintered structure includes the residual austenite, the martensite and the precipitated Cu element.

Even when the member made of the iron base sintered alloy of the present embodiment is used for a long period of time under a high temperature environment, the chronological change of the dimension of the member is restricted.

It is conceivable that when the Cu element, which was dissolved in the martensite, is preliminarily precipitated before the usage under the high temperature environment, the Cu element, which was dissolved in the martensite, is restricted from being precipitated during the usage under the high temperature environment, and thus transition of the residual austenite to the martensite caused by easing of compression stress is restricted.

In other word, at least a part of the Cu element that is expected to be precipitated from the martensite during the usage under the high temperature environment is preliminarily precipitated (i.e., reduced) in the sintered structure of the iron base sintered alloy. As a result, when the iron base sintered alloy is applied to the member and the member is used for a long period of time under the high temperature environment, the precipitation of the Cu element from the martensite during the usage under the high temperature environment is suppressed.

Embodiment 2

A method for manufacturing an iron base sintered alloy according to an embodiment 2 will be described.

In the method for manufacturing the iron base sintered alloy according to the present embodiment, a Cu precipitation treatment is conducted for a sintered alloy before the iron base sintered alloy is to be used. The sintered alloy has a chemical composition including at least Fe, Cu and C, and a sintered structure including a residual austenite and a martensite. The Cu precipitation treatment is conducted to preliminarily precipitate Cu element dissolved in the martensite.

The method for manufacturing the iron base sintered alloy according to the present embodiment may be a method for manufacturing the iron base sintered alloy according to the embodiment 1. Therefore, the present embodiment may refer to the embodiment 1.

For example, the chemical composition and the sintered structure of the sintered alloy before the Cu precipitation treatment is conducted may refer to the embodiment 1. For example, the Cu element is preliminarily precipitated in a state of intermetallic compound of Cu. For example, the intermetallic compound of Cu includes Fe—Cu intermetallic compound. These descriptions may also refer to the embodiment 1.

The sintered alloy before conducting the Cu precipitation treatment is provided by molding sintered alloy powder that includes raw material powder for providing a predetermined chemical composition, and then sintering and quenching the obtained molded member. The sintered alloy powder may include lubricant disappearing in the sintering, for example, for the purpose of increasing demolding property from a metal mold. Regarding the method for manufacturing the sintered alloy powder, the method for molding the sintered alloy and the condition of the quenching, well-known method for manufacturing the sintered alloy powder, method for molding and condition may be suitably applied.

Regarding the sintered structure of the sintered alloy before conducting the Cu precipitation treatment, a content of Cu in the martensite is, for example, equal to or greater than 0.5% by mass. In this configuration, the residual austenite is likely to be stabilized by the dissolution of Cu. The content of Cu in the martensite is measured by the TEM-EDX analysis with respect to a sample of the sintered alloy before conducting the Cu precipitation treatment.

Regarding the sintered structure of the sintered alloy after conducting the Cu precipitation treatment, a content of Cu in the martensite is, for example, less than 0.5% by mass. The content of Cu in the martensite is measured by the TEM-EDX analysis with respect to a sample of the sintered alloy after conducting the Cu precipitation treatment (i.e., obtained iron base sintered alloy before the usage under high temperature environment).

When the content of Cu in the martensite is less than 0.5% by mass, the precipitation of the Cu element from the martensite during the usage under the high temperature environment is certainly restricted. Therefore, the iron base sintered alloy capable of restricting the chronological change of the dimension of the member is obtained. From viewpoints of securing the above effects, the content of Cu in the martensite is preferably equal to or less than 0.4% by mass. More preferably, the content of Cu in the martensite is equal to or less than 0.3% by mass. Further preferably, the content of Cu in the martensite is equal to or less than 0.2% by mass. Further preferably, the content of Cu in the martensite is equal to or less than 0.15% by mass. Further preferably, the content of Cu in the martensite is equal to or less than 0.1% by mass.

Specifically, the Cu precipitation treatment may be conducted by a thermal treatment. More specifically, the Cu precipitation treatment includes a thermal treatment at a temperature applying thermal energy equivalent to thermal energy that is calculated from a temperature in which a sintered member made of the iron base sintered alloy is used and a life of the sintered member. The thermal treatment is preliminarily conducted at the temperature applying the thermal energy equivalent to the total thermal energy that is to be applied to the sintered member when the sintered member is used at the temperature until the end of the life of the sintered member.

As such, Cu element dissolved in the martensite is preliminarily precipitated in the sintered structure. Accordingly, the iron base sintered alloy capable of restricting the chronological change of the dimension of the member from the beginning of the usage of the member to the end of the life of the member is obtained.

From viewpoints of enhancing the effects of restricting the chronological change of the dimension, the temperature of the Cu precipitation treatment is preferably equal to or greater than 200 degrees Celsius. More preferably, the temperature of the Cu precipitation treatment is equal to or greater than 205 degrees Celsius. Further preferably, the temperature of the Cu precipitation treatment is equal to or greater than 210 degrees Celsius. Further preferably, the temperature of the Cu precipitation treatment is equal to or greater than 215 degrees Celsius. Further preferably, the temperature of the Cu precipitation treatment is equal to or greater than 220 degrees Celsius.

For example, from viewpoints of saturation of the effects of restricting the chronological change of the dimension and energy saving, the temperature of the Cu precipitation treatment is preferably equal to or less than 300 degrees Celsius, and more preferably equal to or less than 250 degrees Celsius.

The method for manufacturing the iron base sintered alloy of the present embodiment is described hereinabove. According to the method for manufacturing the iron base sintered alloy of the present embodiment, the iron base sintered alloy capable of restricting chronological change of the dimension of the member even when the iron base sintered alloy is applied to the member that is to be used for a long period of time under a high temperature environment is obtained. The iron base sintered alloy and the method for manufacturing the iron base sintered alloy will be more specifically described with the following experiment.

<Experiment 1>

A sintered alloy powder is provided by adding Cu powder, black lead and lubricant for demolding the metal mold to Fe—Mo alloy powder prepared by water atomization. The provided sintered alloy powder has a chemical composition including at least Fe, Cu and C except for the component of the lubricant disappearing in the sintering. Specifically, the sintered alloy powder has the chemical composition including 2.0% by mass of Cu, 0.75% by mass of C, 0.45% by mass of Mo, the balance of Fe and inevitable impurities.

Regarding the raw materials, an average particle size d50 of the Fe—Mo alloy powder is 70-80 micrometers (μm), an average particle size d50 of the Cu powder is 15-20 micrometers and an average particle size d50 of the black lead powder is 15-20 micrometers. The average particle size d50 is a particle size (i.e., diameter) when a volume-based cumulative frequency distribution value measured by laser diffraction and scatter method indicates 50%.

The provided sintered alloy powder is molded by a press mold machine to obtain a disc-shaped molded member. The molded member is sintered for 15 minutes at a temperature equal to or greater than 1100 degrees Celsius and equal to or less than 1150 degrees Celsius. Then, the molded member is cooled as a part of the quenching at a speed equal to or greater than 600 degrees Celsius per minute (° C./minute). The Cu element is dissolved in the martensite generated in the quenching. After that, the Cu precipitation treatment is conducted under a predetermined condition for the obtained disc-shaped member made of the sintered alloy to obtain a test sample.

Specifically, a test sample 1 is obtained by the following thermal treatment. The disc-shaped member of the sintered alloy is heated to 225 degrees Celsius at a temperature increasing rate of 3 degrees Celsius per minute. The disc-shaped member is kept at 225 degrees Celsius for 1 hour. Then, the disc-shaped member is cooled to room temperature at a temperature decreasing rate of 3 degrees Celsius per minute.

A test sample 2 is obtained by the following thermal treatment. The disc-shaped member of the sintered alloy is heated to 180 degrees Celsius at a temperature increasing rate of 3 degrees Celsius per minute. The disc-shaped member is kept at 180 degrees Celsius for 1 hour. Then, the disc-shaped member is cooled to room temperature at a temperature decreasing rate of 3 degrees Celsius per minute.

With respect to the test samples 1 and 2, an endurance test is conducted in which the test samples 1 and 2 are kept at 130 degrees Celsius for 100 hours. This endurance test is a simplified simulation of a situation in which the test samples 1 and 2 are used for a long period of time under a high temperature environment.

Hereinafter, the test sample 1 after the Cu precipitation treatment at 225 degrees Celsius may be referred to as an initial test sample 1a, and the test sample 1 after the endurance test may be referred to as a posttest sample 1b. The test sample 2 after the Cu precipitation treatment at 180 degrees Celsius may be referred to as an initial test sample 2a, and the test sample 2 after the endurance test may be referred to as a posttest sample 2b.

The sintered structure of each of the samples is observed by a metallograph of 1000-power magnifications. FIG. 1A shows a metallograph image of a sintered structure of the initial test sample 1a. FIG. 1B shows a metallograph image of a sintered structure of the posttest sample 1b. FIG. 2A shows a metallograph image of a sintered structure of the initial test sample 2a. FIG. 2B shows a metallograph image of a sintered structure of the posttest sample 2b. In the sintered structure shown in FIGS. 1A and 2A, a symbol A indicates the residual austenite, a symbol M indicates the martensite and a symbol P indicates a hole generated in the molding.

With respect to each of the initial test samples 1a and 2a after the Cu precipitation treatment, and the posttest samples 1b and 2b after the endurance test, an absorption spectrum of an energy absorption edge of K-cell of Cu is obtained by X-ray absorption fine structure (XAFS) analysis.

As the measurement instrument, Aichi SR of Aichi synchrotron radiation center is used (beam line: BL11S2, name: Hard X-ray XAFS II, measurement method: Hard X-ray XAFS). Measurement condition is as follows, calibration condition: Cu Fermi level, measurement method: fluorescent yield method, detector: 7ch Silicon Drift Detector (SDD), measurement range: 8684 eV-10000 eV. Measurement results are shown in FIG. 3 and FIG. 4.

According to FIG. 3 and FIG. 4, Cu element, which was dissolved in the martensite, is preliminarily precipitated in the sintered structure of each sintered alloy by the Cu precipitation treatment conducted after the sintering and the quenching. Specifically, the Cu element, which was dissolved in the martensite, is precipitated in a state of intermetallic compound of Cu. More specifically, the Cu element is precipitated in a state of Fe—Cu intermetallic compound. With respect to each of the initial test samples 1a and 2a and posttest samples 1b and 2b, area intensities of the Fe—Cu solid solution and the Fe—Cu intermetallic compound are calculated. The results are shown in Table 1.

TABLE 1 Test sample 1 Peak position Area intensity (225° C. thermal treatment) (angstrom) of peak Initial (before endurance test) Fe—Cu intermetallic compound 2.2578 0.132 Fe—Cu solid solution 2.4870 1.600 After endurance test (130° C. for 100 h) Fe—Cu intermetallic compound 2.2409 0.194 Fe—Cu solid solution 2.2487 1.583 Variation of area intensity Fe—Cu intermetallic compound +0.062 Fe—Cu solid solution −0.017 Test sample 2 Peak position Area intensity (180° C. thermal treatment) (angstrom) of peak Initial (before endurance test) Fe—Cu intermetallic compound 2.2461 0.123 Fe—Cu solid solution 2.4870 1.623 After endurance test (130° C. for 100 h) Fe—Cu intermetallic compound 2.2620 0.196 Fe—Cu solid solution 2.4870 1.583 Variation of area intensity Fe—Cu intermetallic compound +0.073 Fe—Cu solid solution −0.040

According to Table 1, when more Cu element, which was dissolved in the martensite, is precipitated in the sintered structure at a stage of the Cu precipitation treatment after the sintering and the quenching, that is, at a stage before being used under the high temperature environment, variation of the amount of the Fe—Cu solid solution after being used under the high temperature environment is reduced.

Namely, the Cu element, which was dissolved in the martensite, is preliminarily precipitated before being used under the high temperature environment. In this case, the Cu element, which was dissolved in the martensite, is restricted from being precipitated during the usage under the high temperature environment and the transition of the residual austenite to the martensite caused by easing of compression stress is restricted.

According to the results, when the Cu precipitation treatment is conducted by the thermal treatment at the temperature applying a thermal energy equivalent to a thermal energy that is calculated from the usage temperature and the life of the sintered member, the chronological change of the dimension during the usage of the member is restricted.

Next, the temperature of the Cu precipitation treatment conducted after the sintering and the quenching is varied and a variation rate of a dimension at the same point of each of the test samples before and after the endurance test are measured. In the measuring the dimension, 3-dimensional dimension measurement instrument (e.g., XYZAXSVAfusion9/6/6 produced by TOKYO SEIMITSU CO., LTD.) is used. The results are shown in FIG. 5.

According to FIG. 5, as the temperature of the Cu precipitation treatment is increased, the chronological change of the dimension of the member during the usage under the high temperature environment is likely to be restricted. When the temperature of the Cu precipitation treatment is equal to or greater than 210 degrees Celsius, the variation rate of the

dimension is likely to be stabilized at low value. According to the results shown in FIG. 5 and Table 1, when the area intensity of the peak of the Fe—Cu intermetallic compound before the usage under the high temperature environment is equal to or greater than 0.125, and the area intensity of the peak of the Fe—Cu solid solution before the usage under the high temperature environment is equal to or less than 1.620, the iron base sintered alloy capable of restricting the chronological change of the dimension of the member during the usage under the high temperature environment is surely obtained.

Next, with respect to the initial test sample 1a, the posttest sample 1b, the initial test sample 2a and the posttest sample 2b, the content of Cu in the martensite is measured by TEM-EDX analysis.

The content of Cu in the martensite of the initial test sample 1a is 0.1% by mass. The content of Cu in the martensite of the posttest sample 1b. The content of Cu in the martensite of the initial test sample 2a is 0.5% by mass. The content of Cu in the martensite of the posttest sample 2b is 0.1% by mass.

Accordingly, when the content of Cu in the martensite before the usage under the high temperature environment is less than 0.5% by mass, the precipitation of the Cu element from the martensite is surely restricted during the usage under the high temperature environment. Hence, the iron base sintered alloy capable of restricting the chronological change of the dimension of the member is obtained.

While only the selected exemplary embodiment and examples have been chosen to illustrate the present disclosure, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made therein without departing from the scope of the disclosure as defined in the appended claims. Furthermore, the foregoing description of the exemplary embodiment and examples according to the present disclosure is provided for illustration only, and not for the purpose of limiting the disclosure as defined by the appended claims and their equivalents. 

What is claimed is:
 1. An iron base sintered alloy comprising: a chemical composition including at least Fe, Cu and C; and a sintered structure including a residual austenite and a martensite, wherein the sintered structure includes a precipitated Cu element that was dissolved in the martensite and that has been precipitated from the martensite.
 2. The iron base sintered alloy according to claim 1, wherein the precipitated Cu element includes intermetallic compound of Cu.
 3. The iron base sintered alloy according to claim 2, wherein the intermetallic compound of Cu includes Fe—Cu intermetallic compound.
 4. The iron base sintered alloy according to claim 3, wherein the sintered structure shows a peak of the Fe—Cu intermetallic compound at an energy absorption edge of K-cell of Cu in X-ray absorption fine structure analysis, and an area intensity of the peak of the Fe—Cu intermetallic compound is equal to or greater than 0.125.
 5. The iron base sintered alloy according to claim 1, wherein the sintered structure shows a peak of Fe—Cu solid solution at an energy absorption edge of K-cell of Cu in X-ray absorption fine structure analysis, and an area intensity of the peak of the Fe—Cu solid solution is equal to or less than 1.620.
 6. The iron base sintered alloy according to claim 1, wherein a content of Cu in the martensite measured by energy dispersive X-ray analysis with a transmission electron microscope is less than 0.5% by mass.
 7. The iron base sintered alloy according to claim 1, wherein the iron base sintered alloy is to be included in a member for an engine of an automobile.
 8. A method for manufacturing an iron base sintered alloy, the method comprising: conducting a Cu precipitation treatment for a sintered alloy that has a chemical composition including at least Fe, Cu and C, and a sintered structure including a residual austenite and a martensite to precipitate Cu element dissolved in the martensite.
 9. The method for manufacturing the iron base sintered alloy according to claim 8, wherein the Cu precipitation treatment includes a thermal treatment applying a thermal energy, and the thermal energy is equivalent to a thermal energy that is calculated from a temperature in which a sintered member made of the iron base sintered alloy is to be used and a life of the sintered member.
 10. The method for manufacturing the iron base sintered alloy according to claim 8, wherein the Cu precipitation treatment is conducted so that a content of Cu in the martensite measured by energy dispersive X-ray analysis with a transmission electron microscope is less than 0.5% by mass in the sintered structure after the Cu precipitation treatment.
 11. An iron base sintered alloy having a chemical composition including at least Fe, Cu and C, the iron base sintered alloy comprising: a sintered structure including a residual austenite, a martensite and a precipitated Cu element precipitated from the martensite. 